Storage tank

ABSTRACT

A storage tank includes an exterior cover extending over at least a portion of the bottom wall of the tank, but does not cover or enclose the entire tank. The exterior cover is sealed to the tank around the portion of the bottom wall to contain any leakage through the bottom wall portion.

BACKGROUND OF THE INVENTION

This invention generally relates to storage tanks. More specifically, this invention relates to storage tanks with reduced risk of leakage into the ambient soil.

Storage tanks, including above-ground and underground storage tanks, are commonly used for storing liquids, such as gasoline, diesel fuel, heating oil, and the like. Due to increased environmental awareness and associated cleanup costs, leakage from such tanks has become of increased concern.

The most common area where storage tanks, particularly steel tanks, develop leaks is in the bottom wall. Such leaks may result from various factors. One contributing cause of tank leakage has been identified as the use of drop-in measuring sticks or rods which are used to measure the level of liquid within a tank. When a drop-in measuring rod is used to measure the level of liquid, the measuring rod is inserted into an access port in the top of the tank and allowed to drop until the end of the rod contacts or strikes the bottom wall of the tank. It has been found that repeated striking of the measuring rod against the bottom wall (“contact area”) can contribute to the formation of a leak through the bottom wall in the contact area.

Another contributing cause of leakage is an accumulation of water and corrosion-inducing bacteria at the bottom of the tank. It is common for storage tanks to contain some condensation or accumulated water within the tank. Since the water is often denser than the fluid being stored, the water usually settles to the bottom of the tank. This accumulation of water at the bottom of the tank may encourage corrosion and harbor corrosion-inducing bacteria. Some states have regulations limiting the amount of water that may be allowed to accumulate in a fuel storage tank. Regulations may vary from state to state regarding the amount of water allowable in the tank, with some states allowing a certain depth “D” of water, such as up to 6 inches (15.2 cm) or 4 inches (10.2 cm) or up to 2 inches (5.1 cm) or less, and some states allowing up to a certain percentage or amount of water in a test sample.

To reduce the risk of leakage from storage tanks, the industry has sometimes resorted to full double walled tanks. Full double walled tanks have an inner tank for containing liquid and an outer tank for containing any liquid which may have leaked from the inner tank. Since the construction of a full double walled tank essentially requires the manufacturing of two tanks, they are substantially more expensive to manufacture than a single walled tank. Additionally, the extra outer tank makes full double walled tanks heavier and bigger than single walled tanks. This increase in weight and size adversely affects the cost of transporting and installing such tanks.

Accordingly, there remains a need for storage tanks that reduce the risk of leakage while avoiding the costs associated with a full double walled tank.

SUMMARY OF THE INVENTION

The present invention is directed to storage tanks that employ an exterior cover that extends over at least a portion of the bottom wall of the tank, but does not cover or enclose the entire tank, and is sealed to the tank around the bottom wall portion. If a leak should develop in the bottom wall portion enclosed by the exterior cover, for example in the contact area of repeated impact with a measuring rod, the cover will contain the leakage and prevent the liquid from leaking into the ambient soil around or under the tank. Since only a portion of the tank (preferably only a small portion) is enclosed by the exterior cover, leakage protection may be provided without the cost of a full double walled tank.

In accordance with another aspect of the present invention, a sensing apparatus may be provided to detect a leak in the bottom wall of the tank by detecting the presence of liquid between the bottom wall and the cover.

The sensing apparatus may comprise a monitoring pipe communicating between the bottom wall and the exterior cover. In one embodiment, the bottom wall portion of the tank may include an aperture communicating between the bottom wall and exterior cover, and the monitoring pipe (either internal or external to the tank) may extend vertically from the aperture. When a leak occurs in the portion of the bottom wall covered by the exterior cover, the liquid migrates between the bottom wall and the exterior cover, and into the monitoring pipe. An attendant may check for leakage by visually inspecting the monitoring pipe, through an access site, for the presence of liquid. Of course a remote sensor also may be employed in the monitoring pipe or elsewhere to detect the presence of liquid between the bottom wall and the exterior cover.

These and other aspects of the present invention are set forth in the following detailed description. In that respect, it should be noted that the present invention includes a number of different aspects which may have utility alone and/or in combination with other aspects. Accordingly, the above summary is not exhaustive identification of each such aspect that is now or may hereafter be claimed, but represents an overview of the present invention to assist in understanding the more detailed description that follows. The scope of the invention is as set forth in the claims now or hereafter filed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of this description, reference will be made to the accompanying drawings, wherein:

FIG. 1 is a perspective view of one embodiment of a storage tank of the present invention, with portions broken away, and showing a drop-in measuring rod inserted into the tank;

FIG. 2 is a side cross-sectional view of the storage tank shown in FIG. 1;

FIG. 3 is an end cross-sectional view of the storage tank shown in FIG. 1;

FIG. 4 is an enlarged fragmentary cross-sectional view showing the monitoring pipe and bottom wall of the storage tank shown in FIG. 1;

FIG. 5 is a perspective view of another embodiment of a storage tank of the present invention, with portions broken away for better viewing;

FIG. 6 is a perspective view of yet another embodiment of a storage tank of the present invention, with portions broken away;

FIG. 7 is an enlarged fragmentary cross-sectional view showing the storage tank of FIG. 1 with the exterior cover extending over the tank head and the monitoring pipe external to the tank; and

FIG. 8 is an enlarged fragmentary cross-sectional view showing the storage tank of FIG. 1, with the monitoring pipe inserted into an aperture through the bottom wall of the tank.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown one embodiment of a storage tank of the present invention which may be an above-ground or below-ground tank, and which is typical of, but not limited to, underground installation. The storage tank 10 has a main body 12 which may be closed at each end by a tank head 14 to form an enclosed storage volume for stored liquid, such as fuel. The body 12 and the tank heads 14 may be made of steel or any other suitable material for storing liquids and do not need to be separate pieces. When the body 12 and tank heads 14 are made of steel, the tank heads may be welded to the body 12 along welds 13.

The main body 12 may comprise a generally cylindrical shape, which is not necessarily a right circular cylinder. In other words the shape of the body 12 is typically, but not necessarily, circular.

The main tank body 12 includes a top wall or wall portion (“wall”) 16 and a bottom wall or wall portion (“wall”) 17. The top wall 16 preferably has at least one upper access port 18 to allow access to the interior of the tank for various purposes, such as to fill the tank, vent the tank or measure the level of liquid within the tank. As seen in FIGS. 1 and 2, the upper access port 18 may be used to manually measure the level of liquid within the tank 10 by inserting a drop-in measuring rod or stick 20 through the access port 18. The measuring rod 20 is allowed to drop into the tank 10 until the end of the rod 21 contacts or impacts the bottom wall 17 of the tank. The area of the tank that the rod 21 usually impacts is referred to as the contact area.

In accordance with the present invention, as illustrated in FIGS. 1-4 and 7, the tank 10 includes an exterior cover or striker plate 22. The exterior cover 22 may be made of steel or any other formable material, such as a sheet of hard rigid plastic. The exterior cover 22 covers at least a portion of the bottom wall 17, and preferably at least the contact area for tanks using a manual measuring-rod technique. As particularly illustrated in FIG. 2, the exterior cover 22 preferably extends substantially along the length of the tank, stopping short of the welds 13. Alternatively, as seen in FIG. 7, the exterior cover 22 also may cover the tank head 14, extending partially up and over the end of the tank head.

Additionally, the exterior cover 22 may extend around the perimeter of the tank without enclosing the entire tank. Preferably, as illustrated in FIG. 3, the cover 22 may cover a surface of the bottom wall 17 defined by an angle “A”, with angle “A” preferably but not exclusively being between about 5 degrees and about 35 degrees, as measured from the longitudinal axis “X”. In another embodiment, the exterior cover 22 may extend around the perimeter of the tank so that the area of the tank contacted by any selected depth “D” of water in the tank (e.g., 6 inches (15.2 cm) or less, 4 inches (10.2 cm) or less or 2 inches (5.1 cm) or less) is fully enclosed by the exterior cover. It will be understood that the exterior cover may cover the perimeter of the tank up to any selected depth, but not covering the entire tank. It is anticipated that the typical selected depth is about 6 inches (15.2 cm) or less, but this may vary with tank construction or installation conditions and the present invention is not limited to a specific depth.

Preferably, the peripheral edges of the exterior cover 22 are sealed to the tank 10, so that the exterior cover 22 seals and encloses the portion of the bottom wall 17 that is covered by the exterior cover 22. When the tank 10 and the exterior cover 22 are both made of steel, the exterior cover 22 is typically sealed to the tank 10 by welding, as at 23. Alternatively, when the exterior cover 22 is made of some other material, the cover may be sealed to the tank by any suitable type of bonding.

The interstitial space between the exterior cover 22 and the bottom wall 17 forms a leakage containment area to contain any leakage through the portion of the bottom wall 17 covered by the exterior cover 22. The interstitial space may be that small space between the tank wall and cover that results from normal manufacturing tolerances, or the cover may be intentionally spaced from the tank wall to form the interstitial space. Alternatively, channels (such as grooves or other surface variations) may be formed in the cover or tank surface or otherwise provided that allow any leakage from the tank to migrate toward a sensor (which may include an inspection pipe, an electronic sensor or other device for detecting the presence of liquid between the tank wall and cover, as discussed below).

In addition to containing leakage, the storage tank of the present invention may also include a leakage detection system. The leakage detection system may include a sensing apparatus to detect the presence of liquid between the bottom wall 17 of the tank 10 and the exterior cover 22. As shown in FIGS. 1-4 and 8, the sensing apparatus may include a monitoring pipe 26 which communicates with the interstitial space between the bottom wall 17 and the exterior cover 22. In one embodiment, as particularly shown in FIG. 4, an aperture 24 in the bottom wall 17 extends through the bottom wall 17 to communicate between the exterior cover 22 and the bottom wall 17. A monitoring pipe 26 is sealed (e.g., welded) to the bottom wall 17 around the aperture 24 to enclose the aperture such that the monitoring pipe 26 communicates between the bottom wall and the exterior cover via the aperture. Thus, liquid leaking from the tank in the area enclosed by the cover 22 will eventually migrate through the interstitial space, the aperture 24 and into the monitoring pipe. Alternatively, as illustrated in FIG. 8, the aperture 24 may be sized such that the end of the monitoring pipe 26 may be inserted into and through aperture 24 to directly communicate between the exterior cover 22 and the bottom wall 17. In either alternative, the monitoring pipe 26 may vertically extend from the aperture 24 to an upper access site, preferably an above-ground access site for tanks installed underground.

In either embodiment, if a leak were to occur through the portion of the bottom wall 17 covered by the exterior cover 22, the liquid will migrate into the interstitial space between the bottom wall 17 and the exterior cover 22, to the aperture 24 in the bottom wall and into the monitoring pipe 26. The monitoring pipe 26 may contain a remote sensor within the monitoring pipe to detect the presence of liquid. Alternatively, the monitoring pipe 26 may have an access site for visually inspecting the monitoring pipe for the presence of liquid (indicating leakage). Yet another alternative which eliminates the need for an aperture and a monitoring pipe is to place the remote sensor between the bottom wall 17 and exterior cover 22. The sensor may communicate to a read-out instrument via wire or wireless communication.

In an alternative embodiment, the sensing apparatus of the leak detection system may include a monitoring pipe 26 external to the tank which communicates between the exterior cover 22 and the bottom wall 12. As illustrated in FIG. 7, an aperture 28 in the exterior cover 22 extends through the exterior cover to communicate between the wall of the tank and the exterior cover. A monitoring pipe 26, external to the tank 10, may be sealed (e.g., welded) to the exterior cover 22 around the aperture 28 to enclose the aperture such that the monitoring pipe 26 communicates between the exterior cover 22 and bottom wall 17 via the aperture 28. Liquid entering the aperture 28 from the leakage containment area between the exterior cover 22 and the wall of the tank will flow into the monitoring pipe. Alternatively, the monitoring pipe 26 may be inserted into the aperture 28, comparable to FIG. 8, so that the monitoring pipe may directly communicate between the exterior cover 22 and the bottom wall 17. If a leak were to occur, it may be detected by any of the methods or devices previously discussed above.

FIGS. 5 and 6 illustrate alternative embodiments of the present invention which are typical of, but not limited to, above-ground installation. As shown in FIG. 5, the storage tank 210 has a generally cylindrical shaped body 211 which is closed at either end by a tank head 212, which may or may not be a separate piece from the tank body. As illustrated, the tank 210 is vertically installed so that the tank head 212 forms the bottom wall 214 of the tank. The tank 210 also includes an exterior cover 216 that preferably covers substantially the entire bottom wall 214. The exterior cover 216 also may partially extend up the sidewalls 217 of the tank 210, similar to the construction of FIG. 7. The peripheral edges of the exterior cover 214 are sealed to the tank 210 in the manner discussed earlier to form a leakage containment area defined by the size of the exterior cover, and utilizing the interstitial space between the bottom wall 214 and the exterior cover 216.

In order to detect a leak through the bottom wall 214 of the tank 210, an aperture 218 may be provided in the bottom wall 214 which communicates between the bottom wall 214 and the exterior cover 216. A monitoring pipe 220 extending vertically within (or exterior of) the tank is sealed to the bottom wall 214 and is in fluid communication with the aperture 218 so that the monitoring pipe communicates between the bottom wall 214 and the exterior cover 216 via the aperture 218. Alternatively, the monitoring pipe 220 may directly communicate between the bottom wall 214 and the exterior cover 216 by inserting the monitoring pipe into and through the aperture 218, similar to the construction shown in FIG. 8.

When leakage occurs through the bottom wall 214, the liquid will migrate between the bottom wall 214 and the exterior cover 216, to the aperture 218 and into the monitoring pipe 220. A remote sensor may be placed into the monitoring pipe 220 to detect the presence of liquid, or the monitoring pipe 220 may have an access site for visually inspecting the monitoring pipe for the presence of liquid. In the alternative, the aperture and the monitoring pipe may be eliminated and a remote sensor may be placed between the bottom wall 214 and the exterior cover 216, as discussed earlier.

In an alternative embodiment, the monitoring pipe 220 may be external to the tank and be in fluid connection with an aperture which extends through the exterior cover 216 such that the monitoring pipe communicates between the wall of the tank and the exterior cover via the aperture, similar to the construction of FIG. 7. The monitoring pipe 220 also may be inserted into and through the aperture in the exterior cover 216 so that the monitoring pipe directly communicates between the wall of the tank and the exterior cover. A leak may be detected by any of the above mentioned methods or devices.

As seen in FIG. 6, the storage tank 310 also may be of a generally rectangular shape. The tank 310 includes a top wall 312 and a bottom wall 314. The top wall 312 preferably includes at least one upper access port 316 to allow access to the interior of the tank for various purposes, such as measuring the level of liquid in the tank. The tank 310 is shown with a drop-in measuring rod 318 inserted into the access port 316 to manually measure the level of liquid within the tank 310 using the same procedure as discussed earlier.

The tank also includes an exterior cover 320 which preferably extends substantially over the entire bottom wall 314 of the tank 310. The exterior cover 320 also may partially extend up the sidewalls 321 and end walls 323 of the tank 310, similar to the construction of FIG. 7. The peripheral edges of the exterior cover 320 are sealed to the tank 310 in the manner discussed earlier to provide leakage containment in the interstitial space between the bottom wall 314 and the exterior cover 320.

In order to detect a leak through the bottom wall 314 of the tank 310, an aperture 322 in the bottom wall 314 communicates between the bottom wall 314 and the exterior cover 320. A monitoring pipe 324 (interior or exterior to the tank) is sealed to the bottom wall 314 around the aperture 322 such that the monitoring pipe 324 communicates between the bottom wall 314 and the exterior cover 320 via the aperture 322. Alternatively, the monitoring pipe 324 may directly communicate between the bottom wall 314 and the exterior cover 320 by inserting the monitoring pipe into and through the aperture 322, comparable to FIG. 8.

When leakage occurs through the portion of the bottom wall 314 enclosed by the cover, the liquid will migrate between the bottom wall 314 and the exterior cover 320, to the aperture 322 and into the monitoring pipe 324. A sensor may be placed into the monitoring pipe 324 to detect the presence of liquid, or the monitoring pipe 324 may have an access site for visually inspecting the monitoring pipe 324 for the presence of liquid. In the alternative, the monitoring pipe and the aperture may be eliminated and a remote sensor may be placed between the bottom wall 314 and the exterior cover 320, as discussed earlier.

In an alternative embodiment, the monitoring pipe 324 may be exterior to the tank and be in fluid connection with an aperture which extends through the exterior cover 320 so that the monitoring pipe 324 communicates between the wall of the tank and the exterior cover via the aperture. The monitoring pipe 324 also may be inserted into and through the aperture in the exterior cover 320 so that the monitoring pipe directly communicates between the wall of the tank and the exterior cover. A leak may be detected by any of the above mentioned methods or devices.

It will be understood that the embodiments of the present invention which have been described are illustrative of some of the applications of the principles of the present invention. Numerous modifications may be made by those skilled in the art without departing from the spirit and scope of the invention, including those combinations of features that are individually disclosed or claimed herein. 

1. A storage tank comprising: a bottom wall and an exterior cover extending over at least a portion of the bottom wall, but not covering the entire tank, and sealed to the tank around the bottom wall portion to contain any leakage through the bottom wall portion.
 2. The tank according to claim 1 wherein the tank includes side or end walls and said exterior cover extends partially over the side or end walls.
 3. The tank according to claim 1 wherein said tank comprises a generally cylindrical elongated shape having a length and a perimeter and said tank is adapted to be installed horizontally, said exterior cover extending at least substantially along the length and partially along the perimeter of said tank.
 4. The tank according to claim 3 wherein said tank comprises a right cylindrical shape having a longitudinal axis and said exterior cover extends between about 5 degrees and about 35 degrees along the perimeter of the tank as measured from the longitudinal axis.
 5. The tank according to claim 1 wherein said tank comprises a generally cylindrical elongated shape having opposite ends and being closed at each end by a tank head, such that one of the tank heads defines the bottom wall and said cover extends at least substantially over the entire bottom wall.
 6. The tank according to claim 1 wherein said tank comprises a generally rectangular shape and said cover extends at least substantially over the entire bottom wall of said tank.
 7. The tank according to claim 1 wherein said cover is sealed to the bottom wall of the tank.
 8. The tank according to claim 1 wherein said tank and cover are made of steel and the cover is sealed to the tank by a weld.
 9. The tank according to claim 1 wherein said tank includes a sensing apparatus to detect the presence of liquid between the bottom wall and the cover.
 10. The tank according to claim 9 wherein said sensing apparatus comprises a monitoring pipe communicating between the bottom wall and cover.
 11. The tank according to claim 10 wherein the sensing apparatus further includes an aperture in either the bottom wall of the tank or the exterior cover, said aperture communicating between the bottom wall and cover, and the monitoring pipe being in sealed communication with the aperture, such that contents leaking through the bottom wall portion can migrate between the tank and cover to the aperture and into the monitoring pipe for detection.
 12. A storage tank comprising: an upper access opening in the tank for receiving a measuring rod, which measuring rod contacts a bottom wall of the tank generally at a contact area; and a striker plate located on the exterior of the tank and overlying at least the contact area, but not covering the entire tank, said striker plate being sealed to the tank around the contact area.
 13. The tank according to claim 12 wherein the tank includes side or end walls and said striker plate extends partially over the side or end walls.
 14. The tank according to claim 12 wherein said tank comprises a generally cylindrical elongated shape having a length and a perimeter and said tank is adapted to be installed horizontally, said striker plate extending at least substantially along the length and partially along the perimeter of the tank.
 15. The tank according to claim 14 wherein said tank comprises a right cylindrical shape having a longitudinal axis and said striker plate extends between about 5 degrees and about 35 degrees along the perimeter of the tank as measured from the longitudinal axis.
 16. The tank according to claim 12 wherein said tank comprises a generally cylindrical elongated shape having opposite ends and being closed at each end by a tank head, such that one of the tank heads defines the bottom wall and said striker plate extends at least substantially over the entire bottom wall.
 17. The tank according to claim 12 wherein said tank comprises a generally rectangular shape and said striker plate extends at least substantially over the entire bottom wall.
 18. The tank according to claim 12 wherein said striker plate is sealed to the bottom wall of the tank.
 19. The tank according to claim 12 wherein said tank and striker plate are comprised of steel and said striker plate is sealed to said tank by a weld.
 20. The tank according to claim 12 wherein said tank includes a sensing apparatus to detect the presence of liquid between the bottom wall and the striker plate.
 21. The tank according to claim 20 wherein said sensing apparatus comprises a monitoring pipe communicating between the bottom wall and striker plate.
 22. A fuel storage tank comprising: a generally cylindrical elongated shape including a perimeter, a length, a bottom wall and opposite ends; a tank head attached to each end of said tank to close the ends; an external plate covering at least a portion of the bottom wall, extending at least along the length of the tank and stopping short of an attachment area of each head, and sealed to the tank to enclose the bottom wall portion.
 23. The tank according to claim 22 wherein said tank comprises a right cylindrical shape having a longitudinal axis and said external plate extends between about 5 degrees and about 35 degrees along the perimeter of the tank as measured from the longitudinal axis.
 24. The tank according to claim 22 wherein said external plate is sealed to the bottom wall of the tank.
 25. The tank according to claim 22 wherein said tank, tank heads and external plate are made of steel and each of the tank heads are attached to the tank by a weld, and said external plate is sealed to the tank by a weld.
 26. The tank according to claim 22 wherein said tank includes a sensing apparatus to detect the presence of liquid between the bottom wall and the external plate.
 27. The tank according to claim 26 wherein said sensing apparatus comprises a monitoring pipe communicating between the bottom wall and external plate.
 28. The tank according to claim 27 wherein the sensing apparatus further includes an aperture in either the bottom wall of the tank or the external plate, said aperture communicating between the bottom wall and the external plate, and the monitoring pipe being in sealed communication with the aperture, such that contents leaking through the bottom wall portion can migrate between the tank and external plate to the aperture and into the monitoring pipe for detection.
 29. A fuel storage tank comprising: a top wall and a bottom wall; an opening in the top wall for receiving a measuring rod, which measuring rod contacts the bottom wall of the tank generally at a contact area; an external plate covering at least a portion of the bottom wall including the contact area, but not covering the entire tank, said external plate sealed to the tank around the bottom wall portion to contain any leakage through the bottom wall portion; an aperture in either the bottom wall of the tank or the external plate, said aperture communicating between the bottom wall and external plate; and a monitoring pipe being in sealed communication with the aperture, such that contents leaking through the bottom wall portion can migrate between the tank and external plate to the aperture and into the monitoring pipe for detection.
 30. The tank according to claim 29 wherein said tank comprises a cylindrical elongated shape having a perimeter, a length and opposite ends, and a tank head attached to each end to close the ends and said external plate extending substantially along the length of the tank and partially along the perimeter.
 31. The tank according to claim 30 wherein said external plate extends substantially along the length of the tank stopping short of an attachment area of each tank head.
 32. The tank according to claim 30 wherein said external plate extends partially over the tank heads.
 33. The tank according to claim 30 wherein said tank comprises a right cylindrical shape having a longitudinal axis and said external plate extends between about 5 degrees and about 35 degrees along the perimeter of tank as measured from the longitudinal axis.
 34. The tank according to claim 29 wherein said tank comprises a generally cylindrical elongated shape having opposite ends and being closed at each end by a tank head, such that one of the tank heads defines the bottom wall and said external plate extends at least substantially over the entire bottom wall.
 35. The tank according to claim 29 wherein said tank comprises a generally rectangular shape and said external plate extends at least substantially over the entire bottom wall of said tank.
 36. The tank according to claim 29 wherein said external plate is sealed to the bottom wall of the tank.
 37. The tank according to claim 29 wherein said tank and external plate are made of steel and the external plate is sealed to the tank by a weld.
 38. The tank according to 29 wherein the aperture is in the bottom wall of the tank.
 39. The tank according to 29 wherein the aperture is in the external plate.
 40. The tank according to claim 29 wherein said external plate covers a portion of the tank up to a selected depth.
 41. The tank according to claim 40 wherein said depth is less than or equal to about 6 inches (15.2 cm).
 42. A tank comprising: a top wall, a bottom and a height which is measured from the bottom wall to the top wall; an external cover sealed to the tank to contain any leakage through the bottom wall, wherein the cover covers at least all portions of the tank that are in contact with fluid of a selected depth within the tank without covering or enclosing the entire tank, which depth is less than or equal to about one-half the height of the tank.
 43. The tank according to claim 42 wherein the depth is less than or equal to about 6 inches (15.2 cm).
 44. A storage tank comprising: an upper tank wall portion and a lower tank wall portion; an external cover defining a leakage containment area in the lower tank wall portion only. 